A method known as SMART-CUT®, based on implanting atomic species such as hydrogen and/or rare gases and molecular bonding, allows thin films to be produced and assembled on supports. More precisely, implanting atomic species creates a weakened zone and embrittlement in a layer at a depth at which the film is to be detached from a donor wafer. A support or stiffener is attached thereto by molecular bonding. The implanted layer is then transferred by carrying out a treatment, such as a heat or mechanical treatment, to produce cleavage at the weakened zone to detach the transfer layer. The thickness of the thin transfer film is selected in each case, but in general is on the order of a few hundreds or tens of nanometers. The surface obtained can then be polished such as by using a chemical or a mechanical-chemical method. Such a method can produce heterostructures that cannot be obtained by epitaxy alone.
When heat treatments are carried out, such as to facilitate fracture or strengthen the bonding interface, these are conducted at a lower temperature than during epitaxy, and interdiffusion phenomena can be advantageously reduced. This method also allows the remaining portion of the donor wafer left behind after fracturing or detaching, known as the negative, to be recycled, and this is economically beneficial.
In industrial application, the production of a silicon on insulator (SOI) substrate composed of a thin film of monocrystalline silicon electrically insulated from a bulk substrate. In general, the bulk substrate is silicon, and the insulating silicon layer is amorphous silica.
The method is also applicable to a wide range of materials, whether they form the implanted layer (SiC, GaAs, InP, LiNbO3, etc.), the support or stiffener (monocrystalline or polycrystalline silicon, gallium arsenide, polycrystalline indium phosphide, quartz, etc.), or any bonding layer (SiO2, Si3N4, Pd, etc.).
It is also possible to use the method to produce “partial-substrates” intended to receive an additional layer by epitaxial growth on the transferred thin layer. This can provide several advantages:
size: since some substrates are not available in standard industrial sizes, it is thus possible to carry out a method of transferring a thin layer onto a support or stiffener with a larger diameter. In particular, a 4 inch diameter InP film can be transferred onto a 6 inch diameter support so as to remain compatible with 6 inch standard micro-electronics fabrication facilities;
brittleness: the brittleness of certain bulk substrates (for example InP) can cause the substrates and components to break during fabrication and manipulation and may thereby significantly increase production costs. The layer transfer method can advantageously be employed if a stiffener can provide strength to the structure (for example, a thin InP layer on an Si or GaAs support):
cost: the high cost of certain substrates may justify using a layer transfer method to transfer a very thin layer (a few tens of nanometers thick) onto a cheap stiffening substrate; the operation being repeated after recycling the donor wafer (negative);
compliant effect: this term represents a certain adaptability of the transfer layer, particularly as regards dimensions. In this respect, epitaxial growth is known to require a good match between the lattice parameters and thermal expansion coefficients of the substrate assembly and the epitaxial layer. By way of example, on bulk GaAs substrate it is preferred that, the maximum lattice mismatch not exceed about 1%, otherwise stacking defects typically occur in the epitaxial layer. In one embodiment, techniques can be used that have been developed that allow higher mismatches between the lattice parameters, while making a multilayer structure with an epitaxial seed layer that is sufficiently thin to be able to match itself to the characteristics of the epitaxially grown material by deformation.
It has also to be observed that InP as a substrate for the micro-electronics industry is rapidly gaining popularity. Because of its intrinsic properties, InP and its alloys (InGaAs, AlInAs, InGaP, InGaAsP, InGaAsN, etc.) that can be epitaxially grown thereon with lattice matching, allow transistors to be produced with excellent cutoff and transition frequencies. InP technology is thus the most favorable for producing very high speed optical transmission networks. In optoelectronics, emitters and receivers produced using InP technology can function within wavelength ranges that are used in optical telecommunications. Due to this combination of characteristics, this group of materials can wholly integrate the associated photonic functions and electronic functions of control and amplification in the optoelectronics field. Finally, in the field of microwave amplification, the high power or low noise levels developed by high energy mobility (HEMT) field effect transistors produced using InP technology also contributes to the great success of InP technology.
Currently available substrates formed from InP substrates and the like are bulk substrates obtained by ingot preparation techniques. There are two principal techniques for growth by pulling: liquid encapsulated Czochralski, LEC, and vertical gradient freezing, VGF, as well as a variety of variations and improvements.
The production of large, high quality InP crystals, however, is traditionally fraught with difficulties that involve the crystallization properties of the material. Low twin crystal creation energy and low stacking fault energy promote the appearance of defects in the crystalline structure produced, and the density of these defects has typically increased with ingot size.
Incorporating certain impurities into the melt mixture is also known, either to provide N or P type doping or to render the material semi-insulating, which is accomplished by compensation, preferably with iron. Substrates are sliced from said ingots along the desired crystallographic direction, generally (100) or (111). Subsequent mechanical-chemical polishing produces a finished substrate on which epitaxial growth can be carried out. Growing iron-compensated InP by pulling, however, has typically been happened by a physical property, namely the extremely low segregation coefficient of iron in InP [K(Fe)=10−3]. This causes excessive iron incorporation close to the seed as growth commences, followed by depletion of iron in the melt. An iron concentration gradient exists from the head to the tail of the ingot that results in a variation in iron concentration along the ingot. The variation in iron concentration can be as high as one order of magnitude: for example 1016 cm−3 at one end of the ingot axis and 1017 cm−3 at the other end. Compensation of the substrate, and thus its resistivity, will vary substantially depending on its original position in the ingot.
To overcome this problem it is possible to proceed to an a posteriori bulk-substrate compensation. A technique for incorporating iron by ion implantation would potentially irreversibly damage the InP material.
It is known that InP can be compensated using a diffusion technique. Generally, sealed quartz tube diffusion is employed at a high temperature (about 900° C.) with a compound that is rich in iron and phosphorus, providing a vapor pressure of several atmospheres. The presence of phosphorus prevents desorption of the phosphorus component of the InP from the substrate surface.
The thickness of bulk substrates typically imposes very long diffusion times (typically at least 80 hours (h) for a 600 micrometer (μm) substrate). Thus, this technique is not readily compatible with mass production using bulk substrates.
Because of the size, brittleness, or cost considerations mentioned above, or to provide the substrate with a characteristics compatible for epitaxy, a skilled person may wish to use a SMART-CUT® type technique to transfer a thin layer of InP onto a support. This technique has been carried out for unintentionally doped InP layers, or those doped with the usual dopants (S, Sn and Zn), or compensated by the presence of iron (see the article by E. Jalaguier et al in Proc 11th Int Conf InP and Related Materials, pp 26-7 (1999)). In the case of semi-insulating InP compensated with iron or another compensating material, undesirable interactions are observed between the implanted species (typically hydrogen) and the complexes present in the material and involving iron atoms. Thus, improvements in these processes are desired.